The present invention relates generally to measuring characteristics of a web of moving sheet material and, more particularly, to improved methods and apparatus for measuring web thickness and other characteristics of a moving sheet of paper being manufactured by a paper making machine.
Systems for measuring characteristics of moving webs of material, such as paper webs, are well known in the art. For example, paper web thickness is commonly measured by scanning a measurement head transversely back and forth across a moving web. The measurement head includes contact shoes which engage opposite sides of the moving web. The distance between the shoes is measured and is directly related to the sheet thickness or "caliper."
While prior art web measuring apparatus have been effectively used for a substantial period of time, the requirements for the performance of these apparatus have been constantly increasing. For example, in the paper making industry, increased performance requirements have come about, at least in part, due to the following new industry trends: operating paper web making apparatus at higher speeds; producing thinner products to conserve materials, resulting in brittle sheets which are easily torn by contact sensors; use of recycled products that tend to deposit foreign matter on contact sensors; and, higher pulp utilization resulting in sheets having a greater quantity of small wood matter on their surfaces which can cause inaccuracies in thickness determinations and can result in torn sheets if caught by contact sensors.
In view of these new industry trends, there is a need for an improved measuring apparatus which is more accurate, has quicker overall system response time and is more readily useable with the thinner web sheets which are oftentimes manufactured from recycled materials and/or in compliance with higher pulp utilization requirements.